<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div {
            width: 200px;
            height: 200px;
            background-color: paleturquoise;
            border: 1px solid sienna;
            /* 子绝父相 */
            position: relative;
            border-radius: 30px;
            /* overflow: hidden; */
        }

        div>span {
            display: inline-block;
            width: 30px;
            height: 30px;
            background-color: #fff;
            position: absolute;
            /* opacity: .4; */
        }

        div>span:nth-child(1) {
            left: -1px;
            top: -1px;
            border-right: 1px solid sienna;
            border-bottom: 1px solid sienna;
            border-bottom-right-radius: 30px;
        }

        div>span:nth-child(2) {
            right: -1px;
            top: -1px;
            border-left: 1px solid sienna;
            border-bottom: 1px solid sienna;
            border-bottom-left-radius: 30px;
        }

        div>span:nth-child(3) {
            right: -1px;
            bottom: -1px;
            border-left: 1px solid sienna;
            border-top: 1px solid sienna;
            border-top-left-radius: 30px;
        }

        div>span:nth-child(4) {
            left: -1px;
            bottom: -1px;
            border-right: 1px solid sienna;
            border-top: 1px solid sienna;
            border-top-right-radius: 30px;
        }
    </style>
</head>

<body>
    <div>
        <span>上左</span>
        <span>上右</span>
        <span>下右</span>
        <span>下左</span>
    </div>
</body>

</html>